1. Field of the Invention
The present invention relates to zoom lenses and, more particularly, to a zoom lens of the rear focus type having a long back-focal distance for use in film cameras or image pickup apparatus, such as video cameras or broadcasting cameras, while still permitting the relative aperture to be made as large as 1.6 in F-number in the wide-angle end and the range to be extended to as high as 30.
2. Description of Related Art
Recently, home video cameras and the like have reduced their sizes and weights. Along with this there is seen a remarkable advance even in the improvements of the compact form of the zoom lens for the image pickup apparatus. In particular, efforts are being devoted to shortening of the total length of the complete lens, to a reduction of the diameter of the front lens members and to simplification of the configuration.
As one means for achieving these ends, there is known the so-called rear-focus type of zoom lens in which a lens unit other than the front or first one is moved for focusing purposes.
In general, the zoom lenses of the rear focus type have features, as compared with the zoom lens of a type in which the first lens unit is moved to effect focusing, that the first lens unit becomes smaller in effective diameter, making it easier to improve the compact form of the entire lens system, and also that closeup photography, particularly, super-short focusing, becomes easier to do. Further, because the focusing lens unit is relatively small in size and light in weight, a much weaker torque suffices for driving the focusing lens unit. Accordingly, rapid focus adjustment can be performed.
Such a zoom lens of the rear focus type, as disclosed in, for example, Japanese Laid-Open Patent Applications No. Sho 62-215225 (corresponding to U.S. Pat. No. 4,859,042), No. Sho 62-206516, No. Sho 62-24213 (corresponding to U.S. Pat. No. 4,859,042), No. Sho 62-247316, and No. Hei 4-43311, comprises, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, wherein the second lens unit axially moves to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift resulting from the variation of magnification and to effect focusing.
Also, in Japanese Laid-Open Patent Applications No. Hei 4-43311, No. Hei 4-153615, No. Hei 5-19165, No. Hei 5-27167 and No. Hei 5-60973, the fourth lens unit is constructed with one or two positive lenses, so that a zoom lens of short total length is proposed. In Japanese Laid-Open Patent Application No. Hei 5-60974, there is proposed a zoom lens in which the fourth lens unit is constructed with a positive lens and a negative lens, totaling two lenses.
In Japanese Laid-Open Patent Applications No. Sho 55-62419, No. Sho 62-24213, No. Sho 62-215225, No. Sho 56-114920, No. Hei 3-200113, No. Hei 4-242707, No. Hei 4-343313 and No. Hei 5-297275, there are disclosed, in the embodiments thereof, zoom lenses whose third and fourth lens units each are comprised of a positive lens and a negative lens, totaling two lenses.
Further, in Japanese Laid-Open Patent Application No. Hei 3-158813, similarly, there is disclosed a zoom lens comprising, in order from an object side to an image side, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit, in which the second and third lens units are axially moved, while varying the air separation therebetween, to vary the focal length continuously, and a stop is positioned in the space either between the second and third lens units or between the third and fourth lens units and is made movable in unison with the third lens unit.
According to the specification of the above Japanese Laid-Open Patent Application No. Hei 3-158813, it is suggested that, during zooming from the wide-angle end to the telephoto end, the air separation between the second and third lens units decreases. Also, in the embodiments thereof, the third lens unit having the aperture stop takes, in the wide-angle end, a position closest to the image side. Thus, at the wide-angle end or a slightly zoomed position therefrom, where the front lens members admit of a light beam of largest diameter, that neighborhood of the third lens unit which contains the stop is located closest to the image side. The entrance pupil is, therefore, brought to a deep position.
As a result, the zoom lens disclosed in the above Japanese Laid-Open Patent Application No. Hei 3-158813 is disadvantageous at reducing the diameter of the front lens members. Also, the distortion for the wide-angle end becomes large. Accordingly, there is a drawback that it is difficult to extend the range to an ultra high value in good performance while removing the large distortion.
In view of the above examples, there have been proposed a variety of zoom lenses. For example, in Japanese Laid-Open Patent Application No. 3-215810, a zoom lens of the rear-focus type is proposed, comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a stop, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, in which, during zooming from the wide-angle end to the telephoto end, the second lens unit moves toward the image side, while simultaneously moving the stop and the third and fourth lens units in differential relation with their respective loci each being convex toward the object side, and the fourth lens unit is made movable for focusing.
Further, in Japanese Laid-Open Patent Application No. Hei 8-82743, a zoom lens of the rear-focus type having a high range as much as 20 is proposed, comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power having a stop and a fourth lens unit of positive refractive power, totaling four lens units. During zooming from the wide-angle end to the telephoto end, the second lens unit moves toward the image side, while simultaneously moving the stop and the third lens unit in fixed relation with their locus being convex toward the object side and moving the fourth lens unit in a locus convex toward the object side. Focusing is performed by moving the fourth lens unit.
Further, in Japanese Laid-Open Patent Applications No. Hei 8-5913 (corresponding to U.S. Pat. No. 5,847,882) and No. Hei 8-190051 (corresponding to U.S. Pat. No. 5,847,882), five-unit zoom lenses of the rear focus type whose range is about 20 are proposed, comprising, in order from an object side to an image side, a first lens unit of positive refractive power stationary during zooming, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of negative refractive power and a fifth lens unit of positive refractive power stationary during zooming. At least the second and fourth lens units move to effect zooming. The fourth lens unit moves to effect focusing.
Further, in Japanese Laid-Open Patent Applications No. Hei 8-201695 (corresponding to U.S. Pat. No. 5,712,733) and No. Hei 9-21954, four-unit zoom lenses of the rear-focus type are proposed, comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units. The second lens unit moves toward the image side to vary the focal length from the wide-angle end to the telephoto end. The shift of the image plane with zooming is compensated for by moving the fourth lens unit in a locus convex toward the object side. Focusing is performed by moving the fourth lens unit. The third lens unit has a positive lens and a negative lens of meniscus form convex toward the image side in this order from the object side. The fourth lens unit is comprised of a negative lens of meniscus form convex toward the object side and a positive lens in this order from the object side.
In general, when employing the rear-focus type, a zoom lens is made to have such advantages that the entirety of the lens system is improved in compact form, rapid focusing becomes possible and the minimum object distance is easy to shorten.
However, with the back-focal distance secured so long as to accommodate the optical low-pass filter, color filter and others, on the other hand, if it is intended to minimize the variation of aberrations with focusing or to maintain a high optical performance stable throughout the entire focusing range, the appropriate construction and arrangement of constituent lenses of the zoom lens become very difficult to find.
Particularly, for a zoom lens which has secured a large relative aperture and a high range at once, it becomes very difficult to obtain a high optical performance through the entire zooming range and throughout the entire focusing range.